What does "type" mean and is there is a special use for a question mark in ECMA 6?

Question

In React Native Example Code, you'll find at some files the type statement, which encapsulates 4 properties (I'd like to guess), where the last two ones are suffixed with question marks.

type MapRegion = {
  latitude: number,
  longitude: number,
  latitudeDelta?: number,
               ^============   What are these...
  longitudeDelta?: number,
};              ^===========...question marks for?

What does all this mean? In specification of ECMAScript 6 I can't find anything regarding "type".

Ruan Mendes · Accepted Answer · 2015-12-26T20:25:09.640

5

That is flow, a transpiler language that adds static typing to JavaScript.

type MapRegion = {
  latitude: number,
  longitude: number,
  // This property is nullable
  latitudeDelta?: number,
  // This property is nullable
  longitudeDelta?: number,
}; 

// The following does not cause a compilation error

/* @flow */
var a:MapRegion = {
  latitude: 1,
  longitude: 3 
};

edited Dec 26 '15 at 20:25

answered Dec 26 '15 at 14:31

Ruan Mendes

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Do TypeScript scripts have `.js` as file extension? – idmean Dec 26 '15 at 14:33
@idmean, not yet. The default extension is .ts, however there is an item to allow for .js files to be transpiled in one of the future versions. – toskv Dec 26 '15 at 15:17
2

It's flow, not TypeScript. – Felix Kling Dec 26 '15 at 16:49
@FelixKling Thanks, fixed it – Ruan Mendes Dec 26 '15 at 20:18

score 5 · Answer 2 · answered Dec 26 '15 at 16:35

5

Facebook is actually using their version of statically typed transpiled JavaScript called 'Flow'. That is what you are seeing here. It is somewhat similar to TypeScript so they can easily be mistaken for each other.

http://flowtype.org/

answered Dec 26 '15 at 16:35

rmevans9

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What does "type" mean and is there is a special use for a question mark in ECMA 6?

What does all this mean? In specification of ECMAScript 6 I can't find anything regarding "type".

2 Answers2